The present invention relates to a method and system for the storage and delivery of ultra-pure gases for, inter alia, semiconductor processing. In particular, the invention relates to a control vent system for reducing volatile impurities in a gaseous product of ultra-high purity delivered from a storage vessel containing an inventory of a non-cryogenic liquid product and a method and a system for delivering the product from the storage vessel.
The present invention is an improvement over the method and apparatus disclosed in U.S. Pat. No. 5,644,921 ("the '921 patent"), which is incorporated herein by reference. The '921 patent discloses a method and apparatus for storing ultra-high purity non-cryogenic liquefied compressed gases, such as ammonia (NH.sub.3), and delivering a vaporized gaseous product from those liquefied gases for semiconductor processing applications.
Semiconductor manufacturers require high-purity gases and chemicals for production processes to avoid defects in the fabrication of semiconductor devices. Semiconductor fabrication facilities have in the past used electronic grades of process gases supplied in cylinders. The cylinders, which contain liquid volumes of about 40 liters, are installed in gas cabinets, which typically contain one or two such cylinders per station.
As an alternative, users of large volumes of liquefied compressed process gases have met flow demands by pumping liquid product from a storage vessel and vaporizing it prior to use. This technique enables a user to pressurize the delivery system according to their process needs. Although this method is straightforward for large-volume, low-purity users, it is more complicated for large-volume, high-purity users, such as semiconductor fabricators. Chemicals withdrawn from the liquid phase of a storage vessel contain substantially higher levels of metallic and oil contaminants than chemicals withdrawn from the vapor phase of a storage vessel. When the withdrawn liquid is vaporized, the flow stream carries the impurities into the vapor stream to the point of use. Consequently, high-purity users have in the past relied on purifiers to remove the contaminants.
The system disclosed in the '921 patent provides for vapor phase withdrawal of a non-cryogenic liquefied compressed gas (e.g., NH.sub.3) from a bulk delivery source at a high flow rate without collapsing the delivery pressure. Such a vapor phase withdrawal system provides cleaner gas, since the nonvolatile impurities (e.g., oil, metal) and moisture concentrate in the liquid phase. But light impurities, such as atmospherics, partition between the liquid and gas phases of a liquefied compressed gas, whether in a cylinder, drum or tank. In general, the concentration of lights will be higher in the headspace than in the liquid, and as gas is removed form the headspace, more impurity goes into the headspace.
Direct venting of the headspace above a compressed liquid to reduce the volatile impurities is a common practice. This results in a significant loss of product, however, since the usual practice is to vent off 15-20% of a cylinder of gas to reduce the volatile impurities.
It is desired to have a more cost-effective method of delivering a gaseous product of ultra-high purity from a storage vessel containing an inventory of a non-cryogenic liquid product. It is also desired to have an improved storage vessel for reducing volatile impurities in a gaseous product of ultra-high purity delivered from the storage vessel.
It is further desired to have a method and a system which minimize product waste and provide higher purity products.
It is still further desired to have a method of delivering a gaseous product which has a lower cost for abatement.
It is still further desired to have an improved method and system to deliver ultra-pure gases to semiconductor manufacturing processes using an operationally safe, cost-effective, bulk source and delivery system.